In the case of resins used for food containers and the like, excellent processability and mechanical properties are required. Accordingly, there is a continuing need for techniques relating to the preparation of a polyolefin which satisfies a large molecular weight, a broader molecular weight distribution, desirable comonomer distribution and the like, and thus can be preferably used in a container, a bottle cap, or the like since before.
On the other hand, since metallocene catalysts including a Group 4 transition metal are used to easily control the molecular weight and molecular weight distribution of polyolefins, and control a comonomer distribution of polymers, compared to existing Ziegler-Natta catalysts, they have been used in the preparation of polyolefins having improved mechanical properties and processability at the same time. However, there is a drawback that polyolefins prepared using the metallocene catalysts exhibit poor processability because of a narrow molecular weight distribution.
In general, polymers having broad molecular weight distribution exhibit great reduction in viscosity with an increasing shear rate, and thus exhibit excellent processability in the processing area. Polyolefins prepared by metallocene catalysts exhibit high viscosity at a high shear rate due to a relatively narrow molecular weight distribution, etc., and thus there are drawbacks that a high load or pressure is applied during extrusion to reduce extrusion productivity, bubble stability is greatly reduced upon a blow-molding process, and the blow-molded articles have non-uniform surfaces to reduce transparency.
Accordingly, a cascade reactor having a plurality of reactors has been used in order to obtain a polyolefin having a broad multimodal molecular weight distribution using the metallocene catalyst, and attempts have been made to obtain a polyolefin satisfying a broader multimodal molecular weight distribution and a higher molecular weight at the same time through each polymerization step in a plurality of reactors.
However, proper polymerization does not occur in a latter reactor depending on a polymerization time in a former reactor due to high reactivity of the metallocene catalyst. As a result, there were limits to prepare a polyolefin satisfying a sufficiently high molecular weight and a broader multimodal molecular weight distribution at the same time. Accordingly, there is a continuous demand for a technology capable of more effectively preparing a polyolefin which has a high molecular weight and a broader multimodal molecular weight distribution, thereby satisfying mechanical properties and processability at the same time and being preferably used for products.
U.S. Pat. No. 6,180,736 describes a method for producing a polyethylene in a single gas phase reactor or continuous slurry reactor using one metallocene catalyst. When this method is used, there is advantages in that the production cost of a polyethylene is lowered, fouling hardly occurs, and the polymerization activity is stable. In addition, U.S. Pat. No. 6,911,508 describes the production of a polyethylene with improved rheological properties, polymerized in a single gas phase reactor using a new metallocene catalyst compound and 1-hexene as a comonomer. However, the polyethylene produced in the above patents also has disadvantages in that it has a narrow molecular weight distribution, and hardly exhibits sufficient impact strength and processability.
U.S. Pat. No. 4,935,474 describes a method for producing a polyethylene having a broad molecular weight distribution by using two or more metallocene compounds. In addition, U.S. Pat. Nos. 6,841,631 and 6,894,128 describe that polyethylene having a bimodal or multimodal molecular weight distribution is produced by using a metallocene-type catalyst including at least two metal compounds, and thus it can be used in various applications such as films, pipes, hollow molded articles, etc. However, the polyethylene produced in this way has improved processability, but has disadvantages in that, since the distribution state by molecular weight in the unit particle is not uniform, the appearance is rough, and the physical properties are not stable even under relatively good processing conditions.
Given the above circumstances, there is a continuing need for production of more excellent resins, having a balance between various physical properties, or between various physical properties and processability, and further research thereon is needed.